Air-ingress phenomena on the small break in a HTGR are experimentally investigated.

Experiment is investigated for various break sizes, angles, and density ratios.

Maximum air-ingress rate is observed at 120° in break angle.

This study reveals that air-ingress in the small break is governed by; buoyancy and flow inertia.

A non-dimensional parameter is newly proposed to determine the air-ingress flow regimes.

Newly proposed parameter is based on buoyancy versus inertia force.